By way of formwork material for concreting purposes, wood, sheet metal and, more recently, flexible materials such as synthetic plastics material sheets or foamed synthetic materials, have been used. In order, however, to be able to use a flexible formwork skin, it is necessary to maintain the desired formwork shape by using internal spacers of suitable material. By the incorporation of spacers, however, so-called lost formwork, i.e. the synthetic plastics material formwork remains in situ in the concreted components.
On the other hand, so-called telescopic formwork arrangements of steel or light alloy are known, which are used in tunneling work and consist of a multipart formwork system arranged on a mobile assembly. The use of this kind of system makes it possible to build a tunnel lining in a continuous manner. The use of formwork sections of this kind or formwork elements which do not have a uniform, smooth surface, necessitates considerable subsequent work because the concrete surface has to have the flash witnesses removed from it. The use of formwork removal agents which are applied to the formwork before the concreting operations are commenced, although ensuring trouble-free removal of the formwork nevertheless means that subsequent treatment of the surface, at least at the points of connection, is essential.
Also known are tubelike or sacklike formwork arrangements for the concreting of ceilings and particularly for reinforced concrete ribbed ceilings, these arrangements being made of a slack, flexible material such as rubber tubing or sacking. Tubes of this kind can be operated by filling them with air, water or sand and providing a closable insert.
The tubes consist of a sheet of synthetic plastics material as the envelope with tensile tensioning inserts and stiffening inserts which are applied to fixing points and in this fashion ensure that a specific cross-sectional shape is maintained. Even under conditions of intensive maintenance, however, devices made of flexible material are subjected to relatively heavy wear because the tensioning inserts and reinforcing inserts make it very difficult to introduce the devices into position and align them and it is extremely difficult to re-use such hollow formwork elements. A great deal of extra labour is thus involved and efforts have therefore been made to use hollow elements of inexpensive material in order to use them at lost formwork. Quite apart from this, however, the compressive resistance of this kind of tube of synthetic plastics material foil, is too low to justify regarding such an element as a fullgrade piece of formwork.